1. Field of the Invention
The present invention relates to an optical apparatus, and in particular to an optical apparatus composing an optical transmitting/receiving module where a transmitter portion and a receiver portion are integrated into an optical transmission device.
2. Description of the Related Art
Recently, a large-capacity and compact integrated optical transmitting/receiving module has been more and more standardized, and a market of MSA (Multi Source Agreement) optical module such as 300 pin/XENPAK/X2/XFP in 10 Gbps has been expanded.
FIG. 9 shows an arrangement of an optical apparatus generally known as a 10G MSA XFP optical module. This optical apparatus normally includes a single chip CDR (Clock Data Recovery) 1 commonly for a transmitter portion and a receiver portion. The CDR 1 includes therein a CDR portion (not shown) for each of the transmitter portion and the receiver portion. The CDR of the transmitter portion performs a retiming and a regenerating from an electrical signal of transmitted data TX. In the transmitter portion, an LD driver 2 and a transmitter optical sub-assembly (TOSA) 3 are connected in series to the output of the CDR 1. The output of the CDR 1 drives the LD driver 2, and then undergoes an electro/opto conversion in the transmitter optical sub-assembly 3 to be outputted as optical data.
In the receiver portion, a receiver optical sub-assembly (ROSA) 4 and an amplifier 5 are connected in series to the CDR 1. An optical data input to the receiver optical sub-assembly 4 undergoes an opto/electro conversion, and the converted data is further amplified and reshaped by the amplifier 5 to be provided to the single chip CDR 1. In the CDR 1, the retiming and the regenerating are performed from the electrical signal received to be outputted as an electrical signal of received data RX.
On the other hand, in a digital subscriber line (xDSL) transmission, there is a digital subscriber line transmission method, a transmission device, and a transmission system in which, upon training performed prior to data communication, timing information specifying a period when influence of crosstalks is received from an adjoining line is included in a training symbol string, the training symbol string is notified from a station side device to a subscriber device, and the timing information is inserted into the training symbol string by changing a phase of a training symbol (see e.g. patent document 1).
Also, there is a wireless communication terminal, performing a wireless communication in synchronization with a base station, which includes reception strength measurement means measuring a strength of a signal received from the base station, and a transmission timing determination means hastening a transmission timing of a signal transmitted to the base station when the reception signal strength is determined to be weak, based on the result of a comparison between the reception signal strength measured by the reception strength measurement means and a preset value (see e.g. patent document 2).
[Patent document 1] Japanese Patent Application Laid-open No. 2003-23414
[Patent document 2] Japanese Patent Application Laid-open No. 2002-262356
In case of the optical apparatus shown in FIG. 9, for need of a high-density mounting, the single chip CDR 1 can be used. For mounting the single chip CDR 1 on a compact package, the transmitter portion and the receiver portion are arranged close to each other on the same printed board.
At this time, as shown in FIG. 10A, a received signal of a minute amplitude in the order of several tens of millivolt (mV) outputted from the receiver optical sub-assembly 4 is influenced by crosstalks CT from the LD driver 2 which drives the transmitted data TX with a larger amplitude in the order of several volt (V), so that the receiver sensitivity degrades. The crosstalks increase as the transmitter portion and the receiver portion come close to each other. Therefore, the crosstalks become remarkable as the bit rate is enhanced and the optical apparatus becomes compact, so that how the crosstalks are reduced is a problem in a circuit design and a mounting design.
Such crosstalks can be conceived as follows.
In a system where the transmitted data synchronizes with the received data, a phase relationship between the transmitted data and the received data within the optical apparatus is normally fixed. On the other hand, when long-distance optical fiber transmission lines L1 and L2 are connected to a synchronous system 20 as shown in FIG. 10A, and when a fluctuation or the like of a propagation time exists due to an environmental variation, the phase relationship varies at a slow speed in the order of several seconds. Together with this variation, a phenomenon occurs that a receiver sensitivity degradation varies due to the phase relationship of the transmitted data and the received data, even if the above-mentioned crosstalks are constant.
For example, when the changing points of the transmitted data TX and the received data RX within the optical apparatus are coincident with each other as shown in FIG. 10B, the receiver sensitivity degradation seldom occurs. However, when the changing points of the transmitted data TX are positioned around the center of an eye (opening) of the received data RX as shown in FIG. 10C, the receiver sensitivity degradation is maximized.
When such an optical apparatus is used for a non-synchronous system, the phase relationship of the transmitted data and the received data changes at an extremely high speed, so that the receiver sensitivity degradation is averaged on a time basis, and no variation is observed.
However, in the synchronous system as shown in FIG. 10A, the receiver sensitivity degradation varies on a time basis as mentioned above. As shown in FIG. 11, a BER (Bit Error Rate) of the received data for an optical input power changes at random from the minimum receiver sensitivity degradation Dmin (characteristic shown by a mark ●) to the maximum receiver sensitivity degradation Dmax (characteristic shown by a mark ▪) in the absence of the transmitted data TX (characteristic shown by a mark ▴), resulting in variation phenomenon of the BER on a time basis.
Therefore, at the time of designing the optical apparatus and testing the characteristic of the products, it is necessary to suppress the crosstalks between the transmitter portion and the receiver portion so that the criteria of the minimum input sensitivity may be satisfied even in the phase relationship of the transmitted data and the received data where the receiver sensitivity degradation becomes maximum.